Standups are easy prey to the accusation that they love the sound of their own voice. But only Reggie Watts loves it enough to sample it, loop it, and sing songs to his own backing track. By the end of the gig, that's getting self-indulgent. Earlier, the big-haired US beatboxer/comedian – performing as part of Yoko Ono's Meltdown festival – cuts an endearingly strange figure, his stream-of-consciousness mouth-music and non-sequitur humour a step ahead of our efforts to make sense of him. But we catch up: the closing stages yield fewer surprises, as it seems that just as Watts's voice is on a loop, so the joke is, too.

He is best when his vocal looper is one trick among many, rather than the focus of the show. Watts starts by playing chicken with our expectations, at first pretending to be from Croydon, then pretending it's 2008. One non sequitur follows another, as he takes pleasure in making himself impossible to pin down. The big-man dancing is lovable. The songs – semi-improvisatory noodles; songs about songs – draw attention to their own ridiculousness. There's an overly polite anti-racist hymn – "No more racism/ No, thank you/ I've had enough today" – and one choice moment when he mismatches blissed-out chords on a keyboard to a daft dumbshow of heavy‑rock histrionics.

Packing an extraordinary soul singer's voice, Watts is more interested in sound than words; the lyrics are sometimes nonsense, and sometimes frustratingly inaudible. It's great to see humour forged from pure sound. But the more dominated the gig gets by voice and sonic tricks, the less light-footed it is. I prefer the comic at the top of the show, with a scope as wide as his imagination, to the one at the end, imprisoned by his own effects box.

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